Plain bearing composite material

ABSTRACT

The invention relates to a plain bearing composite material provided with a metallic support layer, optionally with a porous carrier layer applied thereto, and with a lead-free sliding layer, which forms a sliding partner and whose sliding layer material is based on plastic. The aim of the invention is to provide a plain bearing composite material that has a long serviceable life when used a high temperatures. To this end the sliding layer material comprises PEEK as a matrix forming plastic constituent, a lubricant provided in the form of zinc sulfide, a hardening constituent provided in the form of titanium dioxide, and additionally comprises carbon fibers. The weight percentage proportion of the lubricant and of the hardening constituent with regard to the mass of the sliding layer material ranges from 5 to 15% by weight, and the lubricant and the hardening constituent are provided in the form of fine particles having a particle size D 50 -value of no greater than 500 nm.

The invention concerns a sliding bearing composite material with ametallic support layer, an optional porous carrier layer disposedthereon, and a lead-free sliding layer which forms a sliding surface fora sliding partner, having a sliding layer material on the basis ofplastic, with PEEK and a lubricant in the form of zinc sulfide and/orbarium sulfate.

A sliding bearing composite material of this type is disclosed in DE 3601 569 A1. This reference mentions PEEK as one of several polymers,although none of the embodiments comprises PEEK. The document teachesthe use of 5-40 vol % of zinc sulfide and/or barium sulfate in thesliding layer material to increase wear resistance. It mentionsadditives, such as glass fibers, glass beads, carbon fibers, ceramicfibers and aramide fibers to increase stability. All embodiments includeglass fibers or glass beads.

DE 37 36 292 A1 discloses a sliding layer film which can be separatelyproduced and which can be applied either directly onto a metallicsupport layer or via an intermediate layer which serves as a bondingagent. Suitable materials for the sliding layer are fluorinated plastic,in particular PTFE, modified PTFE, polyimide, or PEEK. None of theembodiments comprises PEEK. In accordance with this document, one ormore fillers may be added to the sliding layer to increase and/orimprove the thermal conductivity and/or wear properties. In particularcarbon, aluminium oxide, ceramic materials, glass, bronze, molybdenumdisulfide or silicon carbide can be embedded, in dependence on theapplication.

Sliding bearings of sliding bearing composite materials comprising asliding layer on the basis of plastic are widely used in technology forthe most varied of reasons, i.e. with regard to the loading capacity,chemical resistance and/or temperature resistance. Thermoplasticmaterials are known and available which guarantee thermal stability onlyfor operating temperatures of up to approximately 90° C., which includee.g. ABS, high-pressure polyethylene (HD-PE), PVC, polysulfone (PS) etc.There are also a number of so-called technical thermoplastic materialswhich are suitable for operating temperatures of up to approximately150° C., such as e.g. POM, PET, PA.

The present invention concerns such sliding bearing composite materialswhich are suitable for use at continuous operating temperatures of morethan 180° C. They should also have very good tribological properties andfavorable characteristic mechanical values with regard to shapability aswell as high resistance to chemicals. The sliding bearing compositematerials must moreover be suited for manufacture in an industrialproduction process.

These objects are achieved by a sliding bearing composite materialhaving the features of claim 1.

The invention has shown that the particle size of the zinc sulfidelubricant and the titanium dioxide hardening component which are addedto the matrix-forming plastic component, are of great importance. Usingfine particles in the region subject to wear, a “dense” homogeneousdistribution of these substances in the plastic matrix can be obtained.The performance of the sliding bearing composite material can beimproved with regard to low wear rates and also a favorable coefficientof friction. The above-mentioned hardening component and the lubricantof the sliding layer material are preferably present in the form of fineparticles with a D50 particle size value of maximally 400 nm, preferably100 to 350 nm. The above-mentioned D50 value of the particle sizedesignates a particle size, with which 50 weight % of the relevantmaterial have a larger particle size and 50 weight % have a smallerparticle size. Since the added lubricant particles and the particles ofthe titanium dioxide hardening component are powdery particles which areto be produced by or sorted in accordance with technical methods, thecurve of the particle size distribution will usually be bell-shaped: anapproximately normal distribution. The D50 value of the particle sizewill then be close to the maximum of the bell-shaped distribution curve.In accordance with the present invention, the bell-shaped distributioncurve is preferably such that at least 60%, in particular at least 70%,and most preferably at least 80 weight % of the relevant substance has aparticle size within a particle size range about the bell maximum orabout the D50 value of ±50%, for a D50 value of 330 nm, in a particlesize region of 330 nm±165 nm, i.e. of 165 nm to 495 nm.

Moreover, it has proven to be suitable if the particle size distributionis such that the sum residue in weight % for a screen analysis withvarying mesh width t, in particular between 1 μm and 100 nm, can bedescribed by the following relationship:−(t/d)^(βS=)100·ewherein, in a particularly advantageous manner, the characteristic grainsize d is between 0.34 and 0.54 μm and the forming parameter β of thedistribution is between 2.4 and 3.4. A preferred distribution isdistinguished by a characteristic grain size of 0.440 μm (440 nm) and aforming parameter β of 2.87.

The addition of carbon fibers reinforces the sliding layer of thesliding bearing composite material by increasing its rigidity andsolidity as well as its creeping strength. Carbon fibers also increasethe wear resistance. The thermal conductivity, which is of particularimportance is also improved by the addition of carbon fibers to thesliding layer. These fibers prevent overheating of the sliding layer bydischarging the frictional heat, which is generated during operationdirectly on the surface of the sliding layer, to the inside of thesliding bearing composite material, in particular, to the metalliccomponent of an optionally provided porous carrier layer or directly tothe metallic support layer.

The sliding bearing composite material comprising polyetherether ketone(PEEK) as matrix-forming plastic component, in connection with the otherclaimed components is stable at high temperatures, i.e. it can be usedon a continuous basis at temperatures of more than 180° C., e.g. 190 to250° C. Polyphenylene sulfone (PPS) would, in principle, be suitable asa matrix-forming plastic component of a sliding layer material in viewof its temperature stability of up to 260° C. However, PPS forms aninadequate matrix in view of its retaining capacity, as it tends toburst open during shaping and also does not meet the tribologicalperformance of PEEK.

The present invention does not exclude one or more further thermoplasticmaterials from being contained in the sliding layer material in additionto PEEK as a matrix-forming plastic component. Their portion should notexceed 20 weight %, in particular 10 weight % of the portion of theplastic component in the sliding layer material. The plastic componentis preferably 100% PEEK.

Moreover it has turned out that, in the inventive sliding bearingcomposite material, the addition of PTFE, which is contained inconventional sliding materials in an amount between 2 and 15 weight %,can be omitted even under extreme load conditions. It is assumed thatthe influence of PTFE on the tribological properties of a materialcomposition which is desired per se, is substituted by the claimed zincsulfide component and the alternatively or additionally claimed bariumsulfate component.

In accordance with a preferred embodiment of the invention, the carbonfibers are advantageously short fibers of a length of between 50 and 250μm, in particular 60 to 150 μm. It has turned out that in this case,homogeneous distribution of the carbon fibers in the sliding layermaterial is also obtained within the pores of the optionally providedporous carrier layer, which may e.g. be a bronze layer, in particular alead-tin-bronze layer. This further improves the thermal conductivity byeffectively discharging the produced heat to the porous carrier layer.Carbon fibers having a thickness of 8 to 15 μm have proven to beadvantageous.

The weight percentage portion of the carbon fibers referred to the massof the sliding layer material is preferably 5 to 25 weight %, inparticular 5-15 weight %. It has proven to be advantageous if thesliding layer material has additives of graphite particles in a weightpercentage portion, relative to the mass of the sliding layer material,of 5 to 15 weight %. The graphite particles should preferably be presentas fine particles of a particle size of maximally 15 μm, in particular1-15 μm, preferably 1-5 μm.

It has also turned out that the inventive sliding layer material showsexcellent adhesion to a metallic support layer. The porous carrier layermay consequently be omitted.

The sliding layer composite material can be produced by the followingmethod.

-   -   supplying and pre-heating a strip material forming the carrier        layer,    -   forming a strip-shaped sliding layer material from the        previously mixed and molten sliding layer material through        extrusion of the molten mass,    -   supplying the strip-shaped sliding layer material onto the strip        material forming the carrier layer and joining under pressure at        temperatures of 300 to 500° C.

Extrusion of the plastic sliding material into a thin strip shape anddisposing the strip onto the heated carrier strip produces substantialadvantages. It has turned out that the plastic sliding material can beintroduced into the pores of the porous carrier layer in this mannerwithout previous grinding.

Further features, details and advantages of the invention can beextracted from the claims and the drawing and the following descriptionof material compositions and their properties.

FIG. 1 shows a schematic illustration of various test methods or testbenches;

FIGS. 2 through 4 show a graphic illustration of the measurement ofspecific wear rates under various test conditions using the test methodsof FIG. 1; and

FIGS. 5 through 7 show a graphic illustration of the measurement of thecoefficient of friction for different materials and different testconditions.

The following test results were obtained through different test methodswhich are known per se. Tests (schematically shown in FIG. 1) on aso-called ring-plate test bench (FIG. 1 left-hand side), on a pin-disctest bench/FIG. 1, center) and on a block-ring test bench (FIG. 1,right-hand side) were carried out.

A comparable material designated with P23 was examined wherein thesliding layer material contains PVDF as matrix-forming plastic componentand lead and PTFE as lubricants. Moreover, a sliding bearing compositematerial with a sliding layer of an available sliding layer materialwith PEEK as matrix-forming plastic component and the composition, shownin the following table under PEEK4, was examined. The carbon fibers ofthe material PEEK4 have a length of 1000 μm-5000 μm. ZnS C fibers PTFEGraphite TiO₂ Designation Matrix weight % weight % weight % weight %weight % PEEK4 Rest — 10 10 10 — PEEK PEEK5 Rest 10 10 10 — — PEEK PEEK6Rest 10 10 — 10 10 PEEK PEEK6 Rest 10 — — 10 5 PEEK

The composite materials with designations PEEK 5, 6, 7 were then testedwith sliding layer material compositions which can be gathered from thetable. For all materials, the sliding layer material was introduced intoa porous carrier layer which is sintered onto a steel support layer.

FIGS. 2, 3, and 4 show the test results of the specific wear rate whichwere obtained on a block-ring test bench (FIG. 2, the sliding layermaterial forms the “block”, but it is present in “bulk” form, i.e. noton a porous carrier layer), and on a ring-plate test bench (FIG. 3) or apin-disc test bench (FIG. 4). In the test in accordance with FIG. 2 thefollowing parameters were used: Test duration: 20 h Surface pressure: 1Mpa Sliding speed: 1.0 m/s Counter body: 100Cr6, R_(a) = 0.1-2 μmLubrication: none Room temperature

In the test in accordance with FIG. 3 the following parameters wereused: Test duration: 20 h Sliding speed: 1.57 m/s Counter body: 100Cr6,R_(a) = 0.1-0.2 μm Lubrication: none Room temperature

Measurements were carried out at two different pressures of the ringagainst the plate of 10 N and 20 N.

In the test in accordance with FIG. 4 the following parameters wereused: Test duration: 20 h Surface pressure: 0.5 Mpa Sliding speed: 1.57m/s Counter body: 100Cr6, R_(a) = 0.1-0.2 μm Lubrication: none

Measurement was carried out at two different temperatures, i.e. roomtemperature of 23° C., and 150° C. The measurements show that in thetest of FIG. 2, the inventive sliding bearing composite material PEEK6had a smaller specific wear rate compared to the comparable materials,i.e. has a higher wear resistance. The test of FIG. 3 showed that theinventive sliding bearing composite material PEEK6 is superior at highsurface pressures, i.e. high loads compared to other materials. It showsin particular that omission of carbon fibers and reduction of TiO₂ inPEEK7 had a negative effect on the wear resistance compared to PEEK6,i.e. the wear rate increases. In contrast thereto, addition of TiO₂ toPEEK6 based on PEEK5, which contains no TiO₂ reduces the wear rate.

The test of FIG. 4 showed a superior temperature resistance of theinventive material PEEK6 whereas the other two comparable materials werenot able to meet the requirements and failed, since their sliding layerand a porous carrier layer disposed onto the support layer werecompletely abraded down to the steel support layer.

FIGS. 5 through 7 show tests of the coefficient of friction of theabove-mentioned comparable materials P23, PEEK4 and of the inventivematerial PEEK6.

The test parameters in accordance with FIG. 5 were: Test duration: 20 hSliding speed: 1.57 m/s Counter body: 100Cr6, R_(a) = 0.1-0.2 μmLubrication: none Room temperature

In this test on a ring-plate test bench, the coefficient of friction ofthe materials was measured with two different pressures of 10 and 20 N.

The test parameters in accordance with the test of FIG. 6 on aring-plate test bench were: Test duration: 72 h Sliding speed: 1.57 m/sCounter body: 100Cr6, R_(a) = 0.1-0.2 μm Lubrication: initiallubrication Room temperature

This test of the coefficient of friction differs from that of FIG. 5 inthat initial lubrication was used, whereas the test in accordance withFIG. 5 was carried out without lubrication. The test duration isapproximately three times as long. The inventive material is againsuperior at the higher pressure.

The test parameters of the test of FIG. 7 were: Test duration: 20 hSurface pressure: 0.5 Mpa Sliding speed: 1.57 m/s Counter body: 100Cr6,R_(a) = 0.1-0.2 μm Lubrication: none

A test of the coefficient of friction on a pin-disc test bench at twotest temperatures, i.e. room temperature of 23° C. and an increasedtemperature of 150° C. showed that the inventive material PEEK has asuperior temperature resistance compared to the comparable materials.

A preferred composition of the inventive sliding bearing compositematerial is provided by a support layer of steel and a porous carrierlayer of bronze disposed thereon, e.g. copper-tin-bronze, such asCuSn10, and a sliding layer of a sliding layer material of a compositionin accordance with the embodiment PEEK6, with 10 weight % ZnS, 10 weight% carbon fibers, 10 weight % graphite particles and 10 weight % TiO₂.Zinc sulfide as lubricant and titanium dioxide as hardening substanceare present in the materials PEEK 5, 6, and 7 in the form of extremelyfine particles, wherein the zinc sulfide has an average particle sizewith a D50 value of approximately 300 nm and the titanium dioxide has anaverage particle size with a D50 value of approximately 300 nm. Thisproduces particularly fine homogeneous structures and particularly lowfriction and wear values. The carbon fibers of the PEEK 5, 6, and 7materials have a length of 50 to 250 μm, preferably 60 to 150 μm, andtheir average diameter is 8 to 15 μm. The size of the graphite particlesis up to 15 μm, in particular 1-15 μm, preferably 1-5 μm. PEEK forms therest of the plastic sliding bearing material. Calculated for a volumepercentage composition, the volume percentage portion of the PEEK matrixis preferably between 55 and 90 vol %. A preferred composition consistsof 75 vol % of PEEK4, 4 vol % of ZnS, 10 vol % of short carbon fibers, 7vol % of graphite and 4 vol % of TiO₂.

1-6. (canceled)
 7. A sliding bearing composite material comprising: ametallic support layer; a lead-free sliding layer forming a slidingsurface for a sliding partner, said sliding layer comprising PEEK as amatrix-forming plastic component, a lubricant in the form of zincsulfide and/or barium sulfate, a hardening component in the formtitanium dioxide, and additional carbon fibers, wherein a weight %portion of said lubricant and of said hardening component relative to amass of said sliding layer material is 5-15 weight % and said lubricantand said hardening component are particles having a particle size D50value of not more than 500 nm.
 8. The sliding bearing-composite materialof claim 7, further comprising a porous carrier layer disposed on saidsupport layer.
 9. The sliding bearing composite material of claim 7,wherein said lubricant and said hardening component have a particle sizeD50 value of not more than 400 nm.
 10. The sliding bearing compositematerial of claim 9, wherein said D50 particle size value is 100-350 nm.11. The sliding bearing composite material of claim 7, wherein saidcarbon fibers have a length of 50-250 μm.
 12. The sliding bearingcomposite material of claim 7, wherein said carbon fibers have a lengthof 60-150 μm.
 13. The sliding bearing composite material of claim 7,wherein said carbon fibers have a thickness of 8-15 μm.
 14. The slidingbearing composite material of claim 7, wherein a weight % portion ofsaid carbon fibers relative to said mass of the sliding bearing materialis 5-25 weight %.
 15. The sliding bearing composite material of claim14, wherein a weight % portion of said carbon fibers relative to saidmass of the sliding bearing material is 5-15 weight %.
 16. The slidingbearing composite material of claim 7, further comprising graphiteparticles in a weight % portion of 5-15 weight %, relative to said massof the sliding layer material.